1. Field
The present disclosure relates to ecology fuel return systems, more specifically to shut off valves for ecology fuel return systems.
2. Description of Related Art
Recent environmental protection regulations require the prevention of spillage of any jet fuel and/or jet fuel vapors on runways during aircraft gas turbine engine operation (or after engine shutdown). Most modern aircraft gas turbine engines are equipped with an ecology fuel return system including a compact fuel tank, an ecology valve, tubing, and related components. The return-to-tank fuel ecology system is configured to remove a certain known amount of jet fuel from the engine's fuel manifolds (incl. fuel nozzles/injectors, engine fuel supply lines, and the like) upon engine shutdown. Upon engine startup, the jet fuel from the ecology fuel return system is returned to the engine's fuel pump inlet via an ejector pump to be injected in the combustor (via the fuel nozzles) thus providing stable engine idle operations.
The benefit of the ecology fuel return system is that it minimizes the amount of fuel left over in the engine's fuel system after engine shutdown, thus minimizing the possibility for any liquid fuel and/or any gaseous fuel vapor leaks into the environment. The ecology fuel return system also prevents any potential coking of the fuel manifold nozzles by scavenging the “left-over” liquid jet fuel from the system upon engine shut-down. Finally, the ecology fuel return system drains the combustor of any unused jet fuel upon engine shut-down thus preventing any smoke exhaust form the engine upon engine start-up and potentially causing some localized undesirable fuel-rich conditions in the combustor (i.e., “hot spots”).
However, when a typical ecology fuel return system fails, there is a potential for air leakage into the aircraft's fuel system. Air that is entrained in jet fuel can cause air pockets and uneven jet fuel density leading to uneven jet fuel supply to the engines, potentially resulting reduced thrust power, fuel pump degradation (due to cavitation caused by air pockets), fuel pump damage (due to cavitation in the two-phase flow of gaseous air and liquid fuel) and the like.
To prevent the entrainment of continuous air flow into the fuel system, traditional ecology return systems employ a shutoff valve. This valve is connected to a float to open or close the tank outlet as a function of fluid level. The force from the float's buoyancy acts to close or open the shutoff valve for the tank outlet flow. The float of the shutoff valve is sensitive to the influence of external loads as the valve operates with low or no force margin to keep it closed.
The flow through the inlet shutoff valve creates a pressure drop across that valve that acts to drive traditional valves shut against the weight of the float. As this flow increases, the pressure drop across the valve increases to the point at which it overcomes the mass of the float and armature, thereby prematurely closing the inlet valve prior to fully draining the fuel manifold and nozzles.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved shut off valves for ecology return systems. The present disclosure provides a solution for this need.